Synchronous medullary carcinoma and primary follicular lymphoma in the same thyroid lobe: a case report and literature review

Abstract Primary thyroid lymphoma (PTL) and medullary thyroid carcinoma (MTC) are rare malignancies. Their synchronous development as a collision tumor within a single thyroid lobe is exceptionally uncommon. We present a unique case of a 53-year-old female with a long-standing history of autoimmune thyroiditis who was simultaneously diagnosed with MTC and primary follicular lymphoma (FL, grade 3b) following histological examination of a surgical specimen. The diagnosis was confirmed using immunohistochemistry. Notably, the lymphoma cells lacked BCL2 expression despite a high histological grade, characteristic of a specific localized subgroup of extranodal FLs. This case highlights the critical importance of comprehensive histological and immunohistochemical evaluation of thyroid nodules, particularly in patients with chronic lymphocytic thyroiditis, to guide optimal therapeutic management.